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A Rack middleware component which runs each request in a Fiber from a pool of Fibers.


  • Ruby 1.9
  • EventMachine-based server (e.g. thin or rainbows)


Add a require and use statement to your Rack app. See example/app.rb for a simple Sinatra app which illustrates proper usage. In general, you want the FiberPool to be inserted as early as possible in the middleware pipeline.


You may fix the pool size, otherwise it defaults to 100:

use Rack::FiberPool, :size => 25

All exceptions raised by request handled within a fiber are rescued, by default returning a 500 with no body content. You may customize exceptions rescuing by providing a Proc object conforming to Rack's API:

rescue_exception = { |env, exception| [503, {}, exception.message] }
use Rack::FiberPool, :rescue_exception => rescue_exception


You can see your app's current pipeline of Rack middleware using:

rake middleware

Add Rack::FiberPool to your middleware in config/environment.rb:

require 'rack/fiber_pool' do |config|
  config.middleware.use Rack::FiberPool

If you do experience odd issues, make sure it appears as early in the pipeline as possible. Anything that sets/gets thread local variables (like the Rails' session) or performs I/O should be later in the pipeline. For example, ActionController::Session::CookieStore does not work if it appears before Rack::FiberPool.

You can explicitly place the FiberPool like so:

ActionController::Dispatcher.middleware.insert_before ActionController::Session::CookieStore, Rack::FiberPool

Thanks to

Eric Wong - for adding explicit support for Rack::FiberPool to rainbows.


0.9.3 - fix incompatibility with sinatra streaming, new maintainer (alebsack)


Mike Perham, Twitter, Github, mperham AT